1. Field of the Invention
This invention relates generally to antenna circuits, and more particularly is directed to an improved antenna circuit of the kind including a so-called bar-type antenna.
2. Description of the Prior Art
An antenna circuit is known in the prior art for use in an AM receiver and employs a so-called bar-type antenna constituted by a coil wound around a ferrite core. In such antenna circuit, a field effect transistor (FET) has its source connected to ground, and the coil of the bar-type antenna is connected between the gate of the FET and ground in parallel with a variable condenser. The drain of the FET is connected through a load to a voltage supply source and also to a mixer circuit which further receives a local oscillating signal to provide, as its output, an intermediate frequency signal. However, in such tuned antenna circuit, the inductance of the bar-type antenna has to be adjusted by varying the position of the coil relative to the ferrite core prior to being fixed to the latter. Therefore, the core cannot be directly wound around the ferrite core, but rather has to be wound on a bobbin which is at least initially movable along the core. Further, in order to increase the band width over which tuning can be effected, the Q factor has to be lowered which reduces the sensitivity of the antenna circuit. Further, in the described antenna circuit according to the prior art, phase rotation occurs with a change in the tuning frequency. Moreover, since the described antenna circuit employs a variable condenser for effecting tuning, any difference which is likely to occur between the characteristics of the variable condenser of the antenna circuit and the variable condenser of the local oscillator circuit results in a tracking error which lowers the sensitivity. In a synthesizer type receiver which uses a variable capacitor instead of the variable condenser in the local oscillator circuit, such use of a variable capacitor tends to increase the tracking error.
In order to avoid the above mentioned defects of the first described antenna circuit according to the prior art, it has been proposed to provide an antenna circuit of an aperiodic type which omits the previously described variable condenser. However, in practice, due to the stray capacity of the bar-type antenna and the circuit wiring, the input capacity of the FET and the inductance of the bar-type antenna, resonance occurs in the antenna circuit. In other words, the antenna circuit has a frequency characteristic with a peak at its resonance frequency. By reason of the foregoing, the sensitivity of the antenna circuit varies considerably in accordance with the receiving frequency. If signals, such as, a broadcast wave and the like exist in proximity to the resonance frequency, the multi-signal disturbance characteristic of the circuit is deteriorated. Further, since the gain of the antenna circuit is high at the peak portion of its frequency characteristic, there is a tendency for a parasitic oscillation to occur due to positive feedback.
For the foregoing reasons, in the known antenna circuit of the aperiodic type, it is necessary that the resonance frequency be provided outside the receiving band and that a resistor be provided to damp the peak of the frequency characteristic. However, the foregoing measures lead to other problems. For example, if the resonance frequency is moved outside the receiving band at the high frequency end of the latter, the inductance of the bar-type antenna has to be decreased and this, in turn, lowers the voltage induced in the coil of the bar-type antenna so that the sensitivity thereof is deteriorated. On the other hand, if the resonance frequency is shifted to the lower side of the receiving band, the bar-type antenna becomes capacitive within the receiving band so that the induced voltage is divided by the input capacity of the FET, which again deteriorates the sensitivity. Further, the resistor added to the circuit for damping the peak of the frequency characteristic is a source of noise so that the amount of damping that can be achieved by the added resistor is limited and, as a result thereof, the frequency characteristic cannot be sufficiently flattened.